1. Field of the Invention
This invention relates to a method of and apparatus for reading a radiation image from a radiation image sensor which stores a radiation image upon exposure to radiation bearing thereon the radiation image.
2. Description of the Related Art
Conventionally, for instance, in the medical field, various radiation image sensors on which a radiation image of an object is recorded upon exposure to radiation passing through the object and which outputs electric signals representing the recorded radiation image have been proposed and put into practice.
As the radiation image sensor, there has been known those employing semiconductor material which generates electric charges upon exposure to radiation. As such radiation image sensors, there have been proposed those of so-called an optical-reading system and a TFT system.
As the radiation image sensor of the optical-reading system, a radiation image sensor comprising a first electrode layer permeable to radiation, a recording photoconductive layer which generates electric charges upon exposure to radiation, a charge transfer layer which behaves like a substantially insulating material to the electric charge in the same polarity as a latent image and behaves like a substantially conductive material to the electric charge in the polarity opposite to that of the latent image, a reading photoconductive layer which generates electric charges upon exposure to reading light, and a second electrode layer in which a plurality of linear electrodes permeable to the reading light are arranged in parallel to each other, the layers being superposed one on another in this order, has been proposed.
In the radiation image sensor, radiation carrying thereon a radiation image is exposed from the first electrode layer side and electric charges generated in the recording photoconductive layer are stored on the interface between the recording photoconductive layer and the charge transfer layer, thereby recording a radiation image. When a linear reading light beam scans the radiation image sensor in the longitudinal direction of the linear electrodes, electric charges are generated in the reading photoconductive layer and a part of the electric charges of one polarity is combined with the stored electric charges while the other part of the electric charges of the other polarity flows into the linear electrodes. The electric signal flowing into the linear electrodes are amplified by a reading amplifier connected to the linear electrodes, whereby an image signal is obtained and the radiation image is read out.
In the TFT system, the electric charges stored in the radiation image sensor is read out by selectively turning on and off tow-dimensionally arranged TFT switches.
The electric signal output from the radiation image sensor is digitized by an A/D converter and is obtained as the image signal.
The radiation image sensors described above are disclosed, for instance, in Japanese Unexamined Patent Publication No. 2002-224095.
However, when the electric signal is digitized by an A/D converter, the range of the magnitude of the charge signal stored in the radiation image sensor is sometimes narrower than the conversion range of the A/D converter. In this case, the bit resolution by the A/D converter deteriorates and especially in an electric signal corresponding to a small amount of radiation, quantizing noise is conspicuous since the magnitude of the signal is weak. Though such a problem can be avoided if an A/D converter of high resolution is employed, this approach adds to the cost.